1. Technical Field
The present disclosure generally relates to the electronics field. More in particular, the present disclosure concerns an electronic ignition system for an engine of a vehicle in case of failure during a charging phase of the the coil.
2. Discussion of the Related Art
It is known that most of the electronic ignition systems for the engine of motor vehicles are of the inductive-type. An inductive-type ignition system comprises a battery for supplying energy, a coil having a primary winding with a terminal connected to the battery, a switch connected between the other terminal of the primary winding and ground and a spark plug connected to the secondary winding of the coil. The coil is a transformer which is such to generate a voltage across the secondary winding greater than the voltage across the primary winding.
The inductive-type ignition system operates in the following way. A control unit sends a control signal which closes the switch and starts the charge of the energy into the primary winding: it is generated a current flow from the battery towards ground crossing the primary winding and having an increasing trend, thus storing energy into the primary winding. During the charging phase the control unit calculates the instants wherein the spark at the spark plug can occur, by taking into account information relating the operation of the engine obtained by means of suitable sensors: for example, the instant wherein the spark can occur is when the compression in the cylinder has reached the maximum value. Moreover, the spark can occur only at the instants calculated by the control unit because, otherwise, serious failures in the engine or to different engine components can occur. Therefore at the calculated instants the control unit sends the control signal which opens the switch, which abruptly interrupts the flow of the charge current through the primary winding of the coil, which causes a short length voltage pulse in the primary winding, typically with a peak value of 350-400 V and having a length of few micro-seconds. This voltage pulse generated in the secondary winding of the coil a voltage pulse having a greater value, typically of 35-40 kV, which is sufficient for generating the spark between the electrodes of the plug, so that the air/fuel mixture received in the engine cylinder is burnt.
Moreover, it is known to implement the switch (which enables or interrupts the flow of the charge current through the primary winding) with a Bipolar Junction Transistor (BJT) or with an Insulated Gate Bipolar Transistor (IGBT), which operate in the saturation region when are closed and in the cut off region when they are open (for example, see U.S. Pat. No. 6,807,042 for BJT and U.S. Pat. No. 6,684,867 for IGBT). One of the reasons why it is advantageous to implement the switch with the transistor IGBT is that it is capable of tolerating high currents and voltages of high value (in the example, 350-400 V), typically used in the electronic ignition systems; moreover, the switching rate of the IGBT transistor is lower than the one of other devices (for example, MOSFET), but this is not a limiting factor because the electronic ignition systems use low frequencies.
It is possible that failures occur during the phase of charging the primary winding of the coil. Some examples of failures are the following:
an increase of the value of the temperature internal to the device in which the ignition system is implemented above a threshold value;
over-voltages of the battery voltage or of logic signals (for example, a short-circuit of the control signal with the battery voltage);
a maximum value of the current through a load is reached;
a disconnection of the inter-bonding wires in case the electronic ignition system is implemented with a integrated circuit of the hybrid-type, that is when the controller and the power stage are implemented into different devices which are connected each other with the inter-bonding wires inside the same package.
Protection systems which have the function to shut-down the electronic ignition in case one or more failures occur during the charging phase of the primary winding are known. For example, if the failure is the temperature increase of the device to a value greater than the threshold value, it is necessary to shut-down the device for preventing it from being damaged and thus it is necessary to shut-down the electronic ignition.
The shut-down of the electronic ignition should occur safely, that is it's necessary to discharge the energy stored into the primary winding for preventing the spark from occurring between the plug electrodes at time instants different from those calculated by the control unit; in fact, as previously explained, the spark can occur only at particular instants calculated by the control unit, otherwise serious failures can occur to the engine or to different engine components.
Therefore, if a failure occurs during the charging phase of the primary winding, it is necessary to gradually discharge the energy stored into it by gradually reducing the value of the charge current flowing through the primary winding of the coil and thus by controlling the value of the voltage drop across the primary winding, by keeping at the same time limited the peak values of the voltage drop across the primary winding, in order to avoid the generation of voltage pulses across the secondary winding having abrupt variations and having peak values which are sufficient to generate spurious sparks between the plug electrodes. In the known approaches this is obtained by a linear discharge of the control voltage or current of the switch; consequently, the current flowing through the primary winding of the coil (and thus in the switch) slowly decreases causing the voltage drop across the primary winding to gradually decrease, thus avoiding the generation of spurious sparks. Since the control voltage or current value decrease should be very slow for preventing the generation of the spurious sparks, it is necessary a long time interval before that the value of the charge current through the primary winding of the coil (and thus the value of the voltage drop across the primary winding) starts decreasing and thus a long time interval is necessary (for example, 10-20 milliseconds) from the instant wherein the failure is detected and the time instant wherein the value of the charge current through the primary winding of the coil (and thus the value of the voltage drop across the primary winding) starts decreasing: this time range can be too long and thus it can cause electronic and/or mechanical-type failures (for example, in case of thermal protection the temperature of the device continues to increase during said time interval and the device can be damaged).
In the particular case wherein the switch is implemented with a BJT transistor, it is necessary to extract the current from the base of the BJT transistor towards ground in order to gradually decrease the value of the voltage drop across the primary winding and at the same time to limit the peak value of the voltage drop across the primary winding, by slowly decreasing the value of the charge current flowing through the primary winding (and through the BJT transistor) and increasing the value of the voltage on the collector terminal, in order to prevent the generation of spurious sparks between the electrodes of the spark plug. This known solution has the above mentioned disadvantage of requiring a long time interval between the instant wherein the failure is detected and the time instant wherein the current value through the primary winding of the coil (and thus the value of the voltage drop across the primary winding) starts decreasing.
In the particular case wherein the switch is implemented with an IGBT transistor, it is necessary to control the voltage value at the gate terminal of the IGBT transistor in order to slowly decrease the voltage value at the gate terminal, thus gradually reducing the value of the voltage drop across the primary winding and at the same time limiting the peak value of the voltage drop across the primary winding, thus avoiding the generation of spurious sparks between the electrodes of the plug. This known solution has the previously indicated disadvantage of requiring a long time interval between the instant wherein the failure is detected and the time instant wherein the current value through the coil primary winding (and thus the value of the voltage drop across the primary winding) starts decreasing; during this time interval having a high value the charge current continues to further increase without control till reaching high peak values. Moreover, the known solution with the IGBT transistor has the further disadvantage of not enabling to reliably prevent the generation of the spark (or of spurious sparks) at time instants different from those calculated by the control unit, due to the high transconductance of the IGBT transistor. In fact, small variations of the voltage value at the gate terminal of the IGBT transistor are sufficient to generate a relevant variation of the current value flowing in the collector of the IGBT transistor, thus generating a relevant variation of the voltage at the collector of the IGBT transistor and thus a relevant variation of the voltage drop across the primary winding of the coil, consequently causing a variation of the voltage drop across the secondary winding which is sufficient to generate a spark between the plug electrodes, which can cause serious failures to the engine or to different engine components.
Moreover, the known solutions do not enable to accurately control the decreasing trend of the voltage drop across the primary winding of the coil during the phase of discharging the energy stored into the primary winding after the detection of the failure, with the disadvantage that abrupt variations of the voltage at the primary winding terminal can occur and consequently abrupt variations of the voltage at the terminal of the secondary winding, thus causing undesired sparks between the plug electrodes.